Oil palm plantations in Indonesia are well developed. Based on analysis, it was found that in 2015, the total area of oil palm in Indonesia was 5,980,982 ha, and it increased by about 13.7% by 2017 to 6,798,820 ha [1
]. Various environmental issues related to oil palm commodity production, such as biodiversity, drought, water scarcity, and water and soil resource exploitation, have become major challenges for environmental sustainability. One of the persistent and recurring developing issues is that of water usage. One plant water usage efficiency parameter is the water footprint (WF), which indicates the quantity of water used by plants to produce one mass unit of biomass product. The water footprint of plants consists of a green water footprint (from rainfall), a blue water footprint (from aquifers, rivers, irrigation, etc.), and a gray water footprint (certain quantity of water used to dissolve chemical substances in order to make it appropriate with the environmental threshold) [2
]. The water footprint is generally affected by the water usage of plants and its generated production. Two techniques are used to determine the rate of plant water usage or water utilization, which use a crop water requirement (ETP, potential evapotranspiration) and crop water usage (ETA, actual evapotranspiration) [2
Plant water usage using actual evapotranspiration assumes more representative value to the real condition than to potential evapotranspiration. There are various values of the oil palm water footprint, which are usually based on geographical location and the climate condition. It should also be noted that the value of the water footprint tends to vary based on soil type, plant age, etc.
Nowadays, the water footprint has become an indication of environmental sustainability. There is an urgent need to develop oil palm plantations as a way to sustain plantations and encourage efforts to analyze the water footprint condition in each plantation location. Water footprint analysis could be conducted by various methods, such as the eco-scarcity method, the Milai Canals approach, the Pfister approach, etc. [4
]. The water footprint represents the total sum of water used in a supply chain, which comprises blue, green, and gray water [3
]. Lower water usage input without having a significant impact on yield will decrease the water footprint in milk production [5
However, the limitation of climate data, which is the main factor used to analyze the water footprint in oil palm plantations, has become a major challenge. Moreover, the temporary cultivation of crops, and the various impacts associated with it, have been neglected in analyzing the water footprint in oil palm plantations globally [4
]. Consequently, an annual assessment might be misleading regarding crop choices within and among different regions. A temporal resolution is therefore essential for proper life cycle assessment (LCA) or assessing the water footprint of crop production. For this purpose, a water stress index (WSI) was developed on a monthly basis for more than 11,000 watersheds with global coverage [6
On the other hand, the water footprint has been calculated using an evapotranspiration and productivity approach which gave different ranges of variation between each region [7
]. This analysis was based on geographical location, climate condition, plant condition factors, soil types, etc. As a result of this, and in order to develop the water footprint as a factor of environmental sustainability, a description of the water footprint value in the specific location with various soil types and plant ages is needed. The limitations of climate data for analyzing the water footprint in a specific time frame could be solved by developing a method of water footprint analysis using primary data.
Another factor that affects water usage as the main factor of the water footprint is root density. The oil palm root architecture consists of primary vertical and horizontal roots, secondary horizontal roots, vertical upward and downward growing secondary roots, superficial and deep tertiary roots, and quaternary roots [8
]. The distribution of root density on the structure of oil palm root architecture varies between different soil types and crop ages. Therefore, in this research study, the water footprint of oil palms growing in various soil types and with different plant ages were analyzed. Based on the above-stated problems, it could be stated that variations of oil palm water footprint values are developed based on actual climate and production data in specified locations. It is of interest to find how oil palm water usage varies with different crop ages and soil types, how it distributes between the upper, middle, or lower part of the oil palm root zone, and which one is the highest. Furthermore, it is also important to determine how the water footprint varies temporally and which is greater between consumption from green water from rainwater and blue water from the subsurface layer. The current study is the first to propose the water footprint estimation under varying soil types and crop ages at a specific developmental stage of the oil palm plant in order to provide detailed information about the water footprint of oil palm and as an indicator of environmental sustainability in oil palm plantation.
Site Specific Features of Biophysics and Production
The research was conducted in Pundu village, Central Kalimantan, Indonesia, located at 11°58′01′′ S and 113°04′32′′ E at an altitude of 27 m above sea level. The site experiences a tropical climate and is represented by: (1) average annual rainfall of 3002 mm/year; (2) average annual temperatures between 21.4 and 33.8 °C; and (3) yearly average daily sunshine hours of around 5.9. The various observed plant ages and soil types were obtained from an oil palm plantation of 22,457.7 hectares in area.
Generally, the soil types used for oil palm plantations in Indonesia are spodosols and inceptisols. These soils are spread across Kalimantan and Sumatra island [11
], where most oil palm plantations in Indonesia are located, making it feasible to analyze the effect of the utilization of spodosols and inceptisols and their relation to the water footprint.
There are limiting factors associated with the use of spodosols, such as the depth of the spodic layer, its sandy soil texture, and its acidic texture associated with the tropical area. The depth of the spodic layer is the main factor contributing to poor root growth. This is because it depends on the roots to penetrate the soil, whereas the sandy soil texture will reduce the soil’s ability to retain water and produce a greater chance for the soil to leach its nutrients. Other limiting factors that could possibly hinder plant growth include poor drainage and soil acidity. The depth of the spodic layer in spodosols ranges from 30 to 70 cm below the soil surface [12
]. Oil palm requires solum depths greater than or equal to 80 cm without layers of rock for optimal growth and development [13
]. In some marginal area, the oil palm needs a minimum depth of 75 cm to grow well without additional land improvement [14
Inceptisols are acid mineral soils with low nutrient availability. The productivity of oil palm planted in this soil is low, and there are symptoms of decreased productivity in certain months of the year. The use of inceptisols for agricultural purposes has resulted in many physical, biological, and chemical inconsistency properties of the soil. The problem associated with the physical properties of inceptisols is related to the coarse texture of the topsoil, which happens to be less coarse in the lower layer. Therefore, the permeability value is bigger on the top surface and smaller in the lower layer. The topsoil structure is granular or crumby with a lower unstructured layer. Its density is lower on the surface and increases with depth. The cation exchange capacity is relatively moderate at about 14.1–17.3 me/100 g, while base saturation is low, between the range of 24% to 29% [15
The reference evapotranspiration performed in this study could become the parameter of drought of an area [32
]. Due to the absence of experimental reference evapotranspiration (ETo
) records, the modeling of reference evapotranspiration is reliable usually according to the standard FAO56 Penman Monteith equation (FAO56-PM) [38
]. Based on the result, the average daily reference evapotranspiration obtained from both observation periods had an insignificantly different rate. According to the document of Food and Agricultural Organization FAO no. 56, the average value of ETo for tropical areas, particularly in humid and sub-humid zones ranges between 3 and 5 mm/day for moderate temperature and 5 and 7 mm/day for warm temperatures [17
Actual evapotranspiration in this study referred to oil palm crop water usage, and the actual evapotranspiration is represented by root water uptake. Compared to the study presented in Johor, Malaysia where the annual crop evapotranspiration of oil palm, was calculated to be between 1100 and 1365 mm/year, or similar to 3 to 3.7 mm/day [40
], the result showed in the same range. Additionally, several studies pointed out that the average oil palm crop evapotranspiration was 4.1 mm/day (between 3.5 and 5.5 mm/day) [41
These results can also be compared with other types of plant. For example, the maximum value of daily evapotranspiration varies between 3.3 and 5.6 mm/day for rain-fed sunflower crops and between 6 and 7 mm/day for sunflower crops with optimal irrigation [42
]. Similarly, the evapotranspiration of irrigated sunflower and canola crops varied between 3.6 and 10 mm/day and 2 to 11 mm/day, respectively [43
]. This is similar to the values obtained for oil palm crops, with the consumptive water use of oil palm showing a lower rate.
On the other hand, comparing crop water use with forest plants shows that the level of evapotranspiration of the oil palm is slightly higher. A one-year daily observation in the Bornean tropical rainforest determined a varied evapotranspiration between 2.7 and 2.8 mm/day [44
]. From the analysis obtained, it could be concluded that oil palm is not a crop with an extreme absorption rate that could be categorized as wasteful of water. Even if it could be compared to forest plants in the same location, the water absorption rate is only slightly different.
Root water uptake increases as the plant age increases and as the root becomes denser. The oil palm in spodosol absorbs more water than those in ultisol and inceptisol. This is in line with the root density level shown in Figure 1
b, where the spodosol contains a higher root density than others. This is also supported by the production data in Table 5
, where production over spodosol soil type is higher than in inceptisol. We can also conclude that the highest contribution of root water uptake was in the first root zone, which correlates to the root density distribution.
The findings of this study are also similar to those of one which showed that the root length density and the potential rate of root extraction decreased with the depth of the oil palm root zone [29
]. The soil moisture extraction efficiency (SMEE) value increased with depth and distance from the palm. With regard to other plants, corn field extracted moisture mainly from the upper root dense soil profile when water content was in an optimal range [30
]. Additionally, the distribution and density of wheat roots increases the water uptake [31
Oil palms are often regarded as a plant capable of absorbing a large amount of water, thereby threatening the availability of ground water. From the results obtained in this study, it can be seen that oil palms display a low level of root water uptake when compared to other oil-producing plants, such as sunflower and canola. The distribution of the root water uptake of oil palm plants is mainly from the upper root zone layer. In the first layer, soil moisture comes from rainfall, while in the second and third layers, it comes from either rainwater with deep percolation or the capillary from ground water from a shallow water level with a maximum depth of roots.
The amount of crop water used by the root water uptake is used to analyze the oil palm water footprint with the supported production data listed in Table 4
. The water footprint analysis in this research study is based on a specific location and a partial temporal climate. Studies related to crop water footprints mostly provide the global annual result by ignoring temporal aspects and other influential factors. However, in some areas, such as the Kalimantan region, the temporal aspects and local climate data vary greatly and affect the consumption and use of water as a major factor in the crop water footprint.
The pattern of the crop water footprint changes considerably with higher temporal resolution [6
]. These changes are also shown to be sensitive to crop types due to different growth patterns leading to an increasing or decreasing water footprint. In line with this opinion, the results of this study show that there are variations in water footprint values for various conditions that represent the differences in rainfall, soil type, and growth of oil palm plantations.
The water footprint of the oil palm fresh fruit bunch for the spodosol soil type is lower than that for the inceptisol and ultisol soil types. With the same type of soil, younger plants have a higher water footprint, as shown in Figure 4
. The crop water footprint is mainly driven by yield trends, while evapotranspiration plays a minor role in the annual water crop analysis for the wheat, rice, and maize and soybean footprint. Apart from correlations with yield and irrigation volume, the water footprint values are not correlated to soil properties [45
]; however, it can be seen in Figure 3
that root water uptake varies.
Therefore, if drawn on the annual global scale, there will be a huge significant difference between these variables. The process of root water uptake analysis itself is strongly influenced by climatic factors, soil physical properties, and plant coefficient factors. In this analysis, the discovery of variations in root water uptake and water footprint values at the local and temporal scale could enrich the understanding of the water footprint of oil palm plants in particular, as well as other types of plants.
Another interesting fact worthy of discussion in this research study is the percentage contributions from each element of water (green, blue, and gray) to the total water footprint value of oil palm with variations in age and soil type. As shown in Figure 4
, assuming no fertilization occurred during the observation process, the gray water footprint contribution would be 0%, while the green temporal water footprint reached would be 100% of the total water footprint for almost all variations.
In this oil palm water footprint analysis, the range of blue water footprint is relatively small. In its annual scope, the green, blue, and gray water footprints were 876.6, 35.9, and 91 m3
, respectively, and the contributions were 87.3%, 3.6%, and 9% for the case study in the oil palm plantation in Pundu, Central Borneo [46
]. Additionally, the composition of the green, blue, and gray water footprint to be 68%, 18%, and 14% of the total average of the water footprint from several provinces in Thailand, respectively [47
The crop water footprint for tomato cultivation showed the highest variability found in the water footprint green component, which ranged from 5% to 45.2% [45
]. The blue water footprint ranged from 14.3% to 63.6% and the gray water footprint from 23.8% to 46.5% of the total water footprint. Therefore, it could be said that the range of groundwater use in oil palm plants in this study is relatively small. With no irrigation activities in the field, the possibility of using blue water only comes from the capillarity of groundwater.
The total value of the water footprint, the use of green and blue water, and the distribution of root water uptake in the rooting layers of the oil palm could be described as an indication of environmental sustainability. The various negative issues associated with the absorption of water by oil palm plants are inversely proportional to the results obtained in this study. The root water uptake of the oil palm is relatively low compared to that of other food crops. Additionally, the maximum level of water absorbed in the upper root zone also shows that oil palm plants absorb a lot of rainwater (green water), which is fast circle, compared to ground water (blue water), which is long circle.